A: When you create a new email, the To field takes one contact. You can add more recipients in Copy or Hidden copy. Each of these fields holds up to 150 characters of email addresses in total. If you add more, the email can't be sent until you remove some.
To email many contacts without adding them one by one, select the contacts in your contact list and click Send email in the menu that appears at the bottom of the page. Each contact gets their own copy of the email, addressed only to them, so they won't see that it's been sent to other contacts. You can select up to 100 contacts. To send to more, use email campaigns.